Establishing the Target Population
What is a Target Population?
- Target population denotes the ecological resource about which information is wanted.
Requires a clear, precise definition
- Must be understandable to users
- Field crews must be able to determine if a particular site is included
- More difficult to define than most expect
- Includes definition of the elements that make up, or are associated with, the target population
Also see: Relationships among Target Population, Sample Frame, Samples, Sampled Population, Estimates, and Assumptions
Before the sample survey can be conducted, a clear, concise description of the target population is needed. In statistical terminology the target population (often shortened to "population") does not necessarily refer to a population of people. It could be a population of schools, area units of farm land, freshwater lakes, or the network of streams. The description of the target population must explicitly identify the aquatic resource of interest and include criteria for determining whether a resource unit is in or out of the target population. Another way of thinking about target population is that it is the aquatic resource for which quantitative estimates are needed.
For example, if we were conducting a sample survey to estimate the percentage of students at a university who participate in intramural sports, the target population would consist of all the enrolled students. The individual students would be the sampling units, and the registrar's office could provide a list of students to serve as the sampling frame. We could draw a representative (random) sample of students from this list and interview them about their participation in sports. Their responses would be "yes or no." The percentage of interviewed students who participate in intramural sports would yield an estimate of the "true" percentage for all students.
For a stream survey, the target population might be all perennial, wadeable streams in a watershed. The sampling unit is a point along the stream length, and an associated response design that specifies the measurement protocol and the sampling unit, e.g., electro-shocking with an area 40 times the stream width in length. The response variable might be "degraded" or "non-degraded" based on measures of water quality. Conceptually, the collection of all possible point locations along these streams serve as a sampling frame, similar to the list of students in the previous example.
Lakes - Example
- All lakes (and reservoirs) within the conterminous US, excluding the Laurentain Lakes and the Great Salt Lake with permanent fish populations
- A lake is defined as a permanent body of water of at least one hectare in surface area with a minimum of 1,000 sq m of open (unvegetated) water, and a maximum depth of one meter or more
Streams (Aquatic/Riparian Resource) - Example
- Defined geographic region of interest (State, Region, Ecoregion, Basin, etc)
- Stream channel: habitat, biota, water column
- Stream near-channel: riparian zone(s)
- Stream upland area: terrestrial influence(s)
- Raises question of what constitutes the elements of the target population
Alternative Defining Elements of Stream Target Population
- All watersheds defined by a point anywhere on stream network (Point)
- All watersheds defined by dividing the landscape into hydrologic units at a specific scale (HUC)
- All watersheds defined by stream segments of network (Segment)
Implications of choice
- How many (what proportion of) stream km support aquatic life use?
- How many (what proportion of) watersheds in region have greater than 50% of stream length supporting aquatic life use?
- How many (what proportion of) stream segments in region support aquatic life use?
Estuaries - Example
- Estuarine waters of the Virginia Province
- Large estuaries - those estuaries greater than 260 km2 in surface area and with aspect ratios (i.e., length/average width) of less than 18.
- Large tidal rivers - that portion of the river that is tidally influenced (i.e., detectable tide > 2.5 cm), greater than 260 km2, and with an aspect ratio of greater than 18.
- Small estuaries and small tidal rivers - those systems whose surface areas fell between 2.6 km2 and 260 km2.